1. Field of the Invention
The present invention relates to an engagement structure of a cap unit and a can in a cylindrical type secondary battery and a method of fabricating the same.
2. Description of Related Art
Since secondary batteries have many advantages such as rechargeability, miniaturization, and high capacity, they are frequently used and have become popular. Secondary batteries can be classified into Ni-MH batteries and Li-ion batteries depending on the electrode active material.
It is known that liquid electrolyte, solid polymer electrolyte, or gel type electrolyte are used as the electrolyte of Li-ion secondary batteries. Also, the lithium secondary battery can be classified into a can type or a pouch type depending on the container into which an electrode assembly is inserted.
In a can type lithium secondary battery, the electrode assembly is inserted into a can which is formed by a deep drawing method using metallic materials such as aluminum or an aluminum alloy. Typically, liquid electrolyte is used in a can type secondary battery structure.
Can type secondary batteries can be classified into hexahedronal can types and cylindrical can types. Since the hexahedronal can type can be formed in a thin hexahedronal shape, it can allow installation flexibility when it is used in electronic devices such as a mobile phone. The cylindrical can type is usually used in high capacity electronic devices, in which a plurality of secondary batteries are combined into a single battery pack.
FIG. 1 is a cross-sectional view illustrating an exemplary structure of a conventional cylindrical can type secondary battery.
A method of fabricating a conventional cylindrical can type secondary battery will be described with reference to FIG. 1. First, two electrode plates 25 having a generally rectangular shape and separators 21, 23 interposed between the electrode plates 25 for preventing short circuits between the plates are stacked and wound in a jelly roll configuration to provide an electrode assembly 20. Each electrode plate is formed by coating an active material slurry on a charge collector composed of metallic foil.
The charge collector has uncovered areas absent the active material slurry on either end of the charge collector in a direction in which the electrode plates 21, 23 are wound. On the uncovered areas, electrode taps 27, 29 are provided for each electrode plate. The electrode tap 27 is electrically connected to a cap unit 80. The electrode tap 29 is electrically connected to a cylindrical can 10. The cap unit 80 is insulated from the cylindrical can 10 to provide a path for connecting the electrode assembly to external circuits during charging/discharging of the battery. From the electrode assembly 20, the electrode tap 27 is extracted toward an opening of the cylindrical can 10 and the other electrode tap 29 is extracted in an opposite direction.
The electrode assembly 20 with upper and lower insulation plates 13a, 13b is inserted into the cylindrical can 10 through the can opening. Beads for preventing floating of the electrode assembly in the can are formed, and electrolyte is injected. An insulation gasket 30 is provided around the inside wall of the can opening and a cap unit 80 for covering the can opening is installed inside the gasket 30.
In the cap unit 80, a bent assembly welded to one of the electrode taps 27, a positive thermal coefficient (PTC) element 60, and a cap-up having an electrode terminal are included. The bent assembly typically includes a bent 40 and a current interrupt device (CID) 50 which is used in combination with the bent 40 to cut off a current path.
Subsequently, the side wall of the can opening is pressed toward the center and bottom of the can by using the cap-up 70 inserted into the inside of the gasket 30 as a stopper. In addition, tubing work is performed to form an exterior of the battery.
The electrode tap 27 is formed to have enough length to allow the electrode tap 27 to be welded to the bent 40. Also, the electrode tap 27 is bent while the bent assembly is inserted into the cap opening into which the gasket 30 is already installed. In this process, a small space between the bent assembly and the electrode assembly 20 is provided for process convenience.
After the electrode assembly 20 and the upper insulation plate 13a are inserted into the can, bead areas are provided for fixing these components to the can. However, the bead areas also contribute to formation of a space between the electrode assembly 20 and the cap unit 80. Such spaces are not desired in view of miniaturization and high capacity of the battery.
When the cap unit is installed in the can opening into which the gasket is already inserted and clamping for enveloping the battery is performed, if a high pressure is applied to the side wall of the can opening to improve sealing reliability, the bead areas may be significantly crushed, distorted, or deformed.
The deformation of the bead areas may be greatly varied. When the deformation is significant, the bead areas may be weakened and finally fractured by even a small external force causing air or humidity to flow into the battery or electrolyte to leak.
Also, the deformation of the bead areas may cause irregular heights of each battery module so that the battery module may not sufficiently contact probes of a manufacturing device in subsequent forming processes in which the initial charging and discharging for secondary batteries are executed.